1. Field of the Invention
The present invention relates to polymeric films. More particularly, this invention relates to polymeric films having improved mechanical and gas barrier properties and capable of withstanding retorting.
2. Description of the Prior Art
It is known in the art that polyolefin films, such as polyethylene and polypropylene, are common packaging materials because of their relatively high moisture resistance. However, these polyolefins also have a fairly high permeability to gases, including oxygen, so that if used alone, they are inadequate for packaging oxygen sensitive materials, such as food.
By contrast, polymers and copolymers of vinyl alcohol, such as those of polyvinyl alcohol and ethylene vinyl alcohol, have excellent resistance to gas permeation. However, both ethylene vinyl alcohol and polyvinyl alcohol films tend to lose this desirable property in the presence of moisture. Further, if the vinyl alcohol film is either exposed to high temperatures, such as approximately 240.degree. C. and above, or prolonged heat exposure, the film will form gels and decompose.
It is desirable to sandwich the substantially pure ethylene vinyl alcohol and polyvinyl alcohol compounds between polyolefin layers, but such compounds do not bond well to many polymer films. Furthermore, as the pure vinyl alcohol content of the interior layer is decreased by blending it with other polymers, its oxygen barrier properties likewise fall.
Also commonly used as a component in packaging films are polyamide polymers and copolymers.
Examples of such prior art films are described in U.S. Pat. Nos. 4,254,169; 3,595,740; and 5,055,355.
Another characteristic important to film laminates suitable for packaging materials is the ability to withstand the combination of heat and flexing to which it is often subjected during processes such as pasteurization or sterilization. However, many of the known laminates containing oxygen barrier layers are wholly unsuitable for such procedures in which they are subjected to temperatures between approximately 80.degree. C. to approximately 130.degree. C. As a result of their low softening points, these known barrier laminates are unable to maintain their structural integrity. Other laminates which employ aluminum foil as the barrier component tend to develop pinholes during such procedures, thereby also rendering them unsuitable for such use since such pinholes cause a serious increase in oxygen permeability. Although this tendency can be controlled by sandwiching the foil between two biaxially oriented films, such laminates are inconvenient and costly to produce, and cannot be thermoformed.
It would be desirable to provide a film which has improved mechanical and gas barrier properties and which is capable of withstanding retorting conditions (e.g., temperatures in the range of about 119.degree. C. to 123.degree. C.).